1. Field of the Invention
The present invention relates generally to a SCR system. More specifically, the present invention relates to real-time control of an ammonia distribution grid based on feedback information to provide a zero ammonia slip SCR system.
2. Discussion of the Related Art
The combustion of fossil fuels, such as coal, oil, and industrial or natural gas produces environmentally hazardous substances, including nitric oxide (NO) and nitrogen dioxide (NO2). Nitric oxide and nitrogen dioxide are collectively called nitrogen oxide, or NOx. In the normal combustion process of fossil fuels, the major portion of NOx is NO. The production of NOx can occur when fossil fuel is combusted in a variety of apparatuses, including refinery heaters, gas turbine systems, and boilers, such as in steam plants. The fuel may include coal, oil, gas, waste products, such as municipal solid waste, and a variety of other carbonaceous materials. Federal, state, and regional agencies have established regulations to limit NOx emissions from power plants and other sources.
To comply with governmental regulations, NOx emissions are regulated by combustion controls or utilizing post-combustion methods. The combustion control techniques include boiler tuning, utilization of low NOx burners and/or over-fire air, fuel staging, and other techniques aimed at suppressing NOx formation. These techniques are capable of 25 to 60 percent NOx reduction efficiency. However in many cases, governmental regulations or permits require higher NOx removal efficiency. To accomplish such NOx emissions limits, post-combustion flue gas treatment methods have been commercialized. These methods include selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) processes, combinations of the two processes, and other methods. Higher NOx removal efficiencies (80 to over 90 percent) are possible only when utilizing SCR technology.
Selective catalytic reduction (SCR) reactor technology is used to treat exhaust gases from an industrial process, such as energy production, before the gas is released into the atmosphere. The SCR reactor process relies on the use of a catalyst to treat the exhaust gas as the gas passes through the SCR reactor. Both NOx reducing agent and a catalyst reactor are required for the SCR process to proceed. Because the catalyst is an integral part of the chemical reaction, great effort is used to provide maximum exposure of the catalyst to the exhaust gas and to ensure that all the NOx comes sufficiently into contact with the catalyst and the reducing agent for treatment. In this technology, the SCR catalyst is placed in an optimum temperature window of typically between 550 to 750 degrees Fahrenheit. Because the NOx reducing agent is expensive and consumed in large quantities, new challenging problems need to be addressed concerning reagent utilization and its distribution. If the reducing agent is not entirely consumed in the SCR process, it may be released into the atmosphere. Such release increases the cost of the reagent consumption, resulting in non-optimal utilization of the reducing agent. In addition, governmental regulations limit quantities of the allowable reagent's release into the atmosphere. As a result, proper control of the SCR process requires strict control of both NOx and the reducing agent's release into the atmosphere.
There are a number of known NOx reducing agents. A commonly used NOx reducing agent is ammonia. The principal process for the removal of NOx from the flue gas flow is the injection of a reducing agent, such as ammonia, urea, or any of a number of other known reducing agents, into the flue gas flow. For example, the selective catalytic reduction of NOx involving the injection of ammonia (NH3) into a flue gas flow in the presence of a catalyst occurs as the following chemical reactions:4NO+4NH3+O2→(with catalyst)4N2+6H2O;(main reaction) and2NO2+4NH3+O2→(with catalyst)3N2+6H2O.
The main reaction proceeds over a catalyst layer within a temperature range of 600 degrees Fahrenheit to 750 degrees Fahrenheit. Major components of the catalyst include titanium dioxide (TiO2) and vanadium pentaoxide (V2O5). Additionally, tungsten oxide (WO3) and molybdenum trioxide (MoO3) are added to increase thermal resistance and to limit the deteriorating effects of the catalyst's poisons. Proper selection and sizing of the catalyst volume are critical to achieve the required system performance. Catalyst volume is determined based on catalyst chemical activity, assumed catalyst deactivation rate, deviation of temperature and flue gas flow, and the molar ratio of NH3/NOx across the catalyst bed cross section.
Ammonia, or its precursor, is needed to reduce NOx to innocuous nitrogen and water. According to the above main reaction equation, the reaction between NO and NH3 is equimolar, that is, four molecules of ammonia are required to reduce four molecules of NO. A simple consequence of this reaction mechanism is that in case where more ammonia is supplied than the actual NO concentration, the difference between this oversupplied ammonia and NO leaves the SCR reactor with unreacted ammonia, called ammonia slip. In the reverse scenario, undersupply of ammonia causes NO to escape unreacted (non-reduced) from the SCR catalyst.
One method of injecting ammonia into a flue gas flow utilizes an external ammonia vaporization system in which liquid ammonia (either in anhydrous or aqueous state) is vaporized in a heater or vaporizer, utilizing hot air or flue gas as a carrier gas in the aqueous ammonia case, and then routed to a distribution/injector grid for injection into the flue gas flow at a location “upstream” of an SCR reactor. Because anhydrous ammonia is toxic and hazardous, the general practice is to use a mixture of ammonia and water (NH3.H2O). Ammonia diluted with water, i.e., aqueous ammonia, is less hazardous than anhydrous ammonia.
An ammonia injection grid (AIG) is typically utilized to inject vaporized ammonia into the SCR reactor. Because of the desire in the conventional art to inject a homogenous mixture of flue gas and ammonia into the SCR reactor, the ammonia injection grid is usually located immediately “upstream” from the SCR catalyst reactor (see FIG. 4). In addition to locating the ammonia injection grid immediately before the SCR catalyst reactor, the ammonia injection grid is equipped with jet injectors to further ensure that the ammonia vapor is adequately and evenly distributed across a cross-sectional area, or face, of the catalytic reactor chamber of the SCR system.
With respect to the SCR process, one school of thought is based on the assumption that perfect mixing of all the flue gas components and evenly injecting ammonia would maximize SCR performance. According to this idea, deviations of the NH3/NOx ratio, temperature, and flue gas flow as expressed by a root mean square (RMS) are minimized by initiating some form of mixing process of the flue gas components. However, mixing has a limited effect on reducing flow deviation.
One of the problems related to the usage of an ammonia injection grid is the plugging of the holes (on the grid itself or on the injectors) by the formation of ammonia salts that may occur during operation of the SCR system. To prevent this plugging from disrupting the operation of the SCR system, the diameters of each of the injector holes are typically made to be larger than 3/16 inches. However, larger-sized holes means that fewer holes may be placed in the same area along the pipes of the ammonia injection grid, which reduces the control of the adjustment and distribution of the ammonia injection.
Another problem related to conventional ammonia injection grids utilized in SCR systems is the occurrence of ammonia slip. Ammonia slip occurs when the ammonia that is used as the reducing agent passes through the SCR system un-reacted. Ammonia slip is particularly undesirable because it will ultimately escape into the atmosphere. In conventional SCR systems, ammonia is injected into the reactor chamber in a homogenous mixture with the flue gas throughout the entire cross-sectional area of the reactor chamber. Jet-injector nozzles, for example, may be utilized on the ammonia injection grid to assist in delivery of the ammonia in a homogenous mixture. The injection of a homogenous mixture of ammonia vapor into the SCR chamber actually contributes to the occurrence of ammonia slip.
One instance of when ammonia slip may occur is when too much ammonia is utilized in the reactor chamber. In another instance, ammonia slip occurs due to the uneven, or non-homogeneous, distribution of NOx concentration levels within the reactor chamber. In other words, at any given time within the reactor chamber, different areas of the chamber may have different NOx concentration levels. Therefore, when ammonia vapor is evenly injected into the reactor chamber, in areas of the reactor chamber where there are low NOx concentration or distribution levels, the ammonia yapor passes through mostly un-reacted, causing ammonia slip.
U.S. Pat. No. 5,104,629 to Dreschler discloses a system where ammonia is jet-injected into the intermediate space of a stage of an economizer, where the reducing agent is mixed with low-dust flue gas in the tubes of the economizer. The reducing agent and combustion gas may form a homogenous mixture that passes over catalyst layers downstream of the economizer to effect efficient removal of NOx. Application of the jet-injection and mixing in the economizer tubes does not compensate for velocity mal-distribution, and it only controls ammonia slip emissions to a small degree.
U.S. Pat. No. 5,603,909 to Varner et al. teaches that for optimal performance in an SCR system, uniform flow distribution of ammonia is required and ammonia should be distributed evenly over the flue gas flow cross-section of the boiler. Resulting un-reacted ammonia or ammonia slip passes through the SCR catalyst bed and is collected on the heat exchanger surfaces downstream of the SCR catalyst. The heat exchanger surfaces are periodically washed with water. In this process, ammonia slip is not emitted into the atmosphere, but instead transferred into a liquid solution that requires further processing for disposal. However, the elimination of the occurrence of ammonia slip in the first place is not addressed.
U.S. Pat. No. 4,160,805 to Inaba et al. teaches installing the ammonia injection grid immediately upstream of the SCR catalyst in the temperature field appropriate for the SCR catalyst. The ammonia injection grid is located at a point a little short of the catalyst, in the optimum temperature zone for the catalyst modules. The injection pipes are arranged in a way to uniformly add ammonia to the flue gas. Such an arrangement of the ammonia injection grid does not minimize ammonia slip emissions, primarily due to the occurrence of non-uniform distribution of NOx concentration levels within the reactor chamber.
U.S. Pat. No. 5,988,115 to Anderson et al. discloses a system and method of injecting ammonia in such a manner that a more uniform mixing of the reactant (ammonia) with the flue gas stream is achieved more rapidly, taking advantage of the dynamics of flowing fields exhibiting rotational motion. Anderson et al. teach that in order to ensure optimal SCR operation, it is necessary that the distribution of the reactant across the flue gas stream be as uniform as possible, typically within +/−15 percent of an average value upon entering the SCR chamber. However, as mentioned above, uniform distribution of the reactant does not entirely alleviate the problems with ammonia slip.
U.S. Pat. No. 5,612,010 to Mansour et al. points out that one serious disadvantage involving the SCR process is the risk of unacceptably high levels of ammonia emissions, i.e., high levels of ammonia slip. Mansour et al. recognized that there are technical limitations with SCR performance caused by NOx/NH3 stratification, and they teach an improved integrated selective catalytic reduction/selective non-catalytic reduction (SCR/SNCR) process. According to this SCR/SNCR process, the size of the SCR catalyst is determined based on a pre-selected value of NOx concentration, and ammonia is injected into the SNCR zone when this pre-selected value is exceeded in order to reduce the NOx absolute value. However, problems related to NOx/NH3 stratification are not entirely alleviated because the SNCR process generates even greater mal-distribution of the NOx and NH3 concentrations within the reactor chamber. Although the absolute value of the NOx concentration is ultimately reduced (in the SNCR process), it requires an oversized SCR catalyst to achieve the required performance, as well as installation of an SNCR zone, which is not efficient and has a high capital cost.
Although the prior art has provided SCR system arrangements that are effective for high reduction of NOx concentrations in flue gas, there are problems with implementing control of NOx emissions without emission of unreacted ammonia. The main problem with the simultaneous control of NOx and NH3 emissions steams from the inability to adjust the ammonia concentration profile to the NOx concentration profile at the face of the SCR catalyst. Disparities between the ammonia concentration profile or the NOx concentration profile lead to reduced NOx efficiency (in the case of insufficient ammonia supply) or to emissions of unreacted ammonia (in the case of oversupply of ammonia). This problem is compounded by the fact that the NOx concentration profile is highly non-uniform across the catalyst face and changes with different operating parameters. Moreover, even with homogenous ammonia vapor injection, the problem of ammonia slip still occurs.
In conventional arrangements of the SCR process, ammonia is injected into the flue gas via an ammonia injection grid (AIG) that is typically equipped with multiple injection points. The flow through each injection point is adjusted during the start-up of the SCR system. Such tuning of the grid or setting of the ammonia flow control valve positions is typically performed when operating the boiler at full load. After the valve positions are set, there is no possibility of readjusting the valves without repeating the manual tuning of the grid. The system is considered properly adjusted when 5 to 10 percent of maldistribution between NOx and ammonia exists. However, even with the proper adjustments, maldistribution of NOx and ammonia still exists, which leads to the occurrence of ammonia slip and the escape of unreacted NOx into the atmosphere.
FIG. 1A illustrates a profile of inlet nitrogen oxide levels within a chamber just upstream from SCR catalyst modules. In a typical SCR system, the levels of nitrogen oxide within a chamber of the SCR system are not homogeneous. Different sections or regions within the area of the chamber have different nitrogen oxide levels, measured in parts-per-million corrected (ppmc) to 3% oxygen (in the case of a boiler, it is corrected to 3% oxygen, but in the case of a turbine, it is corrected to 15% oxygen). FIG. 1A illustrates that the NOx concentration was highly maldistributed between 95 to 130 ppmc. The profile of FIG. 1A was obtained from a SCR system running at 310 megawatts (MW) with a 93% NOx reduction efficiency. Ammonia slip was recorded at 10.5 ppmc.
Similarly, as illustrated in FIG. 1B, in which the SCR system was running at a higher load, increased to 318 MW, than in FIG. 1A, the inlet levels of nitrogen oxide within the chamber of the SCR system were also not uniform, and were maldistributed across the cross-sectional area within the chamber reaching concentrations between 145 to 165 ppmc. Notably, a change in the load alone alters the nitrogen oxide patterns, which are not uniformly distributed within the SCR system chamber.
The above-described adjustment of the ammonia injection grid (AIG) and corresponding control of the SCR process presents several more problems. For instance, frequently, boilers are operated by burning different fuels for which combustion utilizes different sets of operating conditions. The NOx concentration profile when burning natural gas may be completely different when operating the same system burning oil, for example. The NOx concentration profile may significantly change with the changing pattern of burners in service or other combustion control equipment, which may change for different boiler loads. In addition, the NOx concentration changes continuously due to fuel quantity, boiler loads, and ambient conditions. The adjustment of ammonia injection is static and pre-established for a particular set of operating parameters, and it is impractical or even impossible to manually change the positions of the valves with continuously changing operating parameters of the boiler.
Accordingly, there is a need for an ammonia distribution grid that provides better control of the adjustment and distribution of ammonia injection, reduces the occurrence of ammonia slip, provides a better operating window, minimizes blockage by ammonia salts, reduces the start-up times of the SCR process, and continuously matches the changing NOx concentration profile with an ammonia concentration profile throughout the duct.